Halogenated compounds are added to polymeric materials to improve flame retardant properties. Such halogenated additives, for example brominated polystyrene, can be effective flame retardants, passing V-O flammability rating in the Underwriters Laboratory UL-94 test for molded bars, and do not seriously impact the mechanical, thermal, ultraviolet photostability, or electrical properties of the flame retarded polymer. However, there are increasing concerns over environmental and toxicological impacts of decomposition products formed when the treated polymer is disposed of or burned. Thus there is a need for an effective halogen-free flame retardant.
Among alternative techniques for improving flame retardancy is the use of additives that protect by intumescence. An intumescence additive when exposed to flame, chars the polymer and generates gases forming a solid foam, creating a barrier between the air and the polymer fuel. Melamine pyrophosphate is used commercially in intumescent paints for this purpose.
Great Britain 2,272,444A discloses a non-halogenated flame retardant comprising a salt of an inorganic phosphorus-containing acid and a polymeric compound having at least ten repeat units. The flame retardant is used as an additive in polymer and coating compositions. A second phosphorus source optionally encapsulated in resin, can be combined with the flame retardant.
A variety of finely divided particulate materials has been added to polymers to modify physical properties, such as pigments, delustrants, anti-tack agents, and to impart electrical conductivity. Such particulate additives have been used in fibers, films, and molded plastics applications. Coating such fine particles prior to dispersion in the polymer has been shown to suppress chemical and photochemical reactivity between the core material of the disperse phase and the continuous polymer phase, and protect the core material from chemical exposure and degradation.
The dispersibility of solid flame retardants in polymers and the tendency for polymer degradation reactions at melt processing temperatures are unsatisfactory. These deficiencies have undesirable processing consequences. The poor dispersion properties necessitate such a high work load in extrusion processing at practical production rates that hot spots are created. In such hot zones decomposition at the polymer/additive interface can produce foaming and inferior compounded product.
The highest performance, and a desirable objective, for flame resistant molded polymers is to pass the V-0 flammability rating in the Underwriters' Laboratory Test UL-94 at a thickness of 1/16 inch (0.16 cm). A high loading of solid flame retardant is needed to meet this requirement. The high loading combined with the poor dispersibility of available solid flame retardants tends to give inconsistent results and to detract from the elongation and strength of polymer.
It is desirable to improve the performance of halogen-free flame retardant additives in polymers, improve the thermal stability of treated polymers, and lower the loading needed to meet the UL-94 V-0 standard for molded structural polymers. Such flame retardant additives are provided by the present invention. It is desirable to make flame retardant concentrates. It is also desirable to have a better process to decrease the high workload required to disperse particulate matter in polymers or to extrude the polymer itself. The present invention provides such a process.